Gas combustion control device

ABSTRACT

For a device of the type in which the amount of fuel gas to be supplied to a gas burner is controlled by a proportional position type solenoid valve, there are provided a first control circuit which controls the exciting current to be applied to the solenoid of the valve in response to the temperature change of a medium heated by said gas burner, thereby controlling the supply of gas fuel to the gas burner, and a second control circuit which detects the magnitude of the exciting current and causes the abrupt change in output current of the first control circuit after the gas burner is ignited and when gas burner is extinguished, thereby causing the abrupt change in gas supply to the gas burner.

United States Patent [191 Mori et al.

[111 3,822,039 [451 July 2,1974

[ GAS COMBUSTION CONTROL DEVICE [75] Inventors: Keijiro Mori, Nara;Keiichi Mori,

Osaka, both of Japan 22 Filed: Ju1y17, 1972 211 Appl. No.: 272,650

[52] US. Cl. 236/75, 236/78 D, 237/8 R,

317/153, 251/131 [51] Int. Cl. H0lh 47/26 I [56] References Cited UNITEDSTATES PATENTS 3,486,693 12/1969 Stang, Jr. et a1 236/78 X PrimaryExaminer--William E. Wayner ABSTRACT For a device of the type in whichthe amount of fuel gas to be supplied to a gas burner is controlled by aproportional position type solenoid valve, there are provided a firstcontrol circuit which controls the exciting current to be applied to thesolenoid of the valve in response to the temperature change of a me diumheated by said gas burner, thereby controlling the supply of gas fuel tothe gas burner, and a second control circuit which detects the magnitudeof the exciting current and causes the abrupt change in output currentof the first control circuit after the gas burner is ignited and whengas burner is extinguished, thereby causing the abrupt change in gassupply to the gas burner.

9 Claims, 9 Drawing Figures PATENIEIJJuL 21914 3.822.039

sum 2 BF 3 EXClTlNG CURRENT TEMPERATURE PATENTEDJIJL 2 m4 mamas GASCOMBUSTION CONTROL DEVICE BACKGROUND OF THE INVENTION The presentinvention relates to a gas combustion control device of the type forcontrolling the combustion rate of a gas burner in proportion to thevariation in load.

In the conventional gas-combustion apparatus such as gas-combustionwater heater for hot water heating, there have been provided a devicefor opening and closing a solenoid valve inserted in a gas supply pipeline in response to the temperature variation of circulating hot wateror a device for controlling a proportional position type solenoid valveinserted in a gas supply pipe line in response to the variation inresistance of a temperature-variable resistor disposed in a hot watersupply main for detecting the temperature variation of circulating hotwater due to the load variation in order to control the gas combustionrate in response to the load variation. In the case of the controldevice for controlling the ON-OFF of the solenoid valve, the combustionis interrupted when the temperature of circulating. hot water risesabove a predetermined level and the combustion is started again when thetemperature falls below the predetermined level. Therefore there arisethe disadvantages that the temperature of circulating hot water isalways subject to variation and that the solenoid valve is easilysusceptible to damage because its ON-OFF operations are very frequentlycycled.

In the latter control device of the type utilizing the proportionalposition type solenoid valve, the gas supply to the gas burner iscontrolled in response to the variation in loadso that the aboveproblems of the temperature variation and of the damages to theelectromagnet valve may be overcome. However, new problems have arisendue to the combustion characteristics of the gas burner and theconstruction of the proportional position type solenoid valve. Ingeneral,v gas issues from the gas nozzle into the air-gas mixturecontroller so that the primary-air is sucked into the air-gas mixturecontroller under the ejector action of gas and mixed with gas. Noproblem arises when gas is supplied in relatively large amounts, butwhen the gas supply is gradually decreased in response to the decreasein load, the kinetic energy of gas issuing from the gas nozzle isdecreased so that the amount of primary air sucked into the air-gasmixture controller is decreased, thus resulting in backfire. Theoccurrence of backfire is generally dependent upon the capacity of thegas burner, and is caused when the amount of gas is reduced to one-halfto one-third of the maximum amount of gas supply.

In the control device of the type utilizing the proportional positiontype-solenoid valve, the plunger supporting means presents a problem. Inthe conventional solenoid valve, the plunger is generally supported by abearing member so that friction is caused between the plunger and thebearing member. In the proportional position type solenoid valve, theplunger is caused to move by controlling the exciting current applied tothe solenoid in response to the variation in load so that the spacingbetween a valve seat and a valve may be suitably adjusted. The spacingbetween the valve and the valve seat is generally of the order of 3 5 mmso that even a small variation in spacing causes a great change in gasflow rate, that is the amount of gas supply to the gas burner.Therefore, the frictional force exerted on the plunger causes the errorin controlling the spacing between the valve and the valve seat, thusresulting in the poor control of gas flow rate in response to thevariation in load.

I SUMMARY OF THE INVENTION The present invention has succeeded inovercoming the above and other problems encountered in the prior artcontrol devices of the type described by electronically interrupting theexciting current applied to the solenoid of the proportional positiontype solenoid valve and by supporting the plunger by leaf springs.

One of the objects of the present invention is therefore to overcome theproblems of backfire and of gas leakage by detecting the temperature ofcirculating fluid heated-by a gas burner by a temperature-variableresistor such as a thermistor and by controlling the exciting current tobe applied to the proportional position type solenoid valve in responseto the variation in resistance of the temperature-variable resistor insuch a manner that the exciting current may be interrupted when itdecreases to less than a predetermined level.

Another object of the present invention is to provide an electronic gascombustion control device which will not supply gas in large quantitywhen the gas burner is ignited again but will gradually increase the gassupply from zero for a short time, thereby preventing the explosionnoise when the gas burner is ignited.

Another object of the present invention is to provide an improvedproportional position type solenoid valve in which a plunger fitted intoa solenoid or coil is supported by a pair of leaf springs, therebyeliminating the frictional force exerted to the plunger in order toattain the precise control of gas flow rate.

Another object of the present invention is to provide an improvedproportional position type solenoid valve in-which a plunger issupported by a pair of leaf springs in such a manner that the solenoidvalve may be prevented from becoming large in size and the displacementof the leaf springs about their fixed ends may be increased, therebypreventing the lateral displacement of the plunger off the axis of itsvertical movement.

Another object of the present invention is to provide an improvedproportional position type solenoid valve in which a plunger issupported by a leaf spring or springs extending from the free end of aleaf spring toward the other end thereof which is fixed to a stationarymember, thereby preventing more positively the lateral displacement ofthe plunger off the axis of the vertical movement thereof.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description ofpreferred embodiments thereof taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a gas-combustion water heaterfor hot water heating to which is applied a gas combustion controldevice in accordance with the present invention;

FIG. 2 is a sectional view of a proportional position type solenoidvalve incorporated in the gas combustion control device of the presentinvention;

FIG. 3 is a perspective view illustrating the plunger supporting means;

FIG. 4 is a perspective view illustrating avariation of the plungersupporting means in accordance with the present invention;

FIG. 5 is a sectional view taken along the line V-V of FIG. 4; 7

FIG. 6 is a perspective view illustrating still another variation of theplunger supporting means in accordance with the present invention;

FIG. 7 is a sectional view taken alongthe line VII- VII of FIG. 6;

FIG. 8 is a circuit diagram of an electronic control unit of the gascombustion control device in accordance with the present invention; and

FIG. 9 is a graph illustrating the relation between the temperature ofhot water detected by a temperaturevariable resistor and the excitingcurrent supplied to the solenoid of the proportional position typeelectromagnet valve.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIGS. 1, 2and 3, a heat exchanger 3 is communicated with a radiator not shown)through a main hot water pipe line 1, through which hot water iscirculated by a circulation pump 4. Hot water at a low temperature isreturned through a return main la, heated by the heat exchanger 3 and issupplied through a supply main lb. A gas nozzle 5 is disposed adjacentto one end 2b of a gas-air mixture controller 2a of a gas burner 2 insuch a manner so that when gas issues from the nozzle 5, the primary airmay be sucked into the gas-air mixture controller 20 under the ejectoraction of the gas. The gas and the primary airare mixed in apredetermined ratio in the controller 2a and the gas-air mixture issuesfrom the nozzles, of the gas burner 2 for combustion. A gas governer 7and a solenoid valve of the proportional position type are inserted intoa gas pipe 6 communicating the gas nozzle 5 with a gas supply source(not shown).

The construction of the solenoid valve 8 is illustrated in detail inFIG. 2. A bobbin 10 carrying a coil .9 is disposed between upper andlower yokes 1,1 and 11a with spacers 12 and 12a interposed therebetween.Supporting members 13 and 13a are struck out of the yoke members 11 and11a respectively as best shown in FIG. 3. A plunger 14 having aprojection 15 which is made of a nonmagnetic material and a diametersmaller than that of the plunger 14, is slidably fitted into a hole ofthe bobbin 10. The bobbin 10 and its associated component parts are allenclosed in a casing 16. The base of a U-shaped leaf spring 17 issecurely fixed to the supporting member 13 struck out of the yoke 11 andthe leading ends of the leg portions of the U-shaped leaf spring 17 arepivoted with pivot pins 14a to the plunger 14 at the upper portionthereof. The leading ends of the leg portion of a U-shaped leaf spring17a are also pivoted to the projection 15 of the plunger 14 with pivotpins 14b, but the base of the leaf spring 17a is fixed to the free endof an adjusting plate 18 whose other end v is securely fixed to thesupporting member 13a struck out of the lower yoke lla. Therefore, theplunger 14 is supported by the pair of upper and lower U-shaped leafsprings 17 and 17a so that the friction between the plunger 14 and thebobbin 10 may be prevented, and the forces of the leaf springs 17 and17a may be used as the retarding forces exerted on the plunger 14 whenthe coil 9 is energized.

' An adjusting plate 18 is provided in order to adjust the force of theleaf spring 170. More particularly, an adjusting screw 19 at the freeend of the adjusting plate 18 may be loosened or tightened so as tocause the free end of the adjusting plate 18 to move toward or away fromthe yoke 11a, thereby changing the initial bending of the leaf springand hence its reaction force. The spring force adjusting plate 18 servesto absorb the influencesv of uneven quality of the leaf springs 17 and17a due to the dimensional errors, and other factors of the leaf springs17 and 17a and of the coil 9 which will adversely affect the smoothmotion of the plunger 14.

A partition disk or cover 20 covers the upper opening of the casing 16,and the upper portion of the plunger 14 extending through the centerhole of the partition disk 20 is sealed with diaphragm 21. A joint 22has connections 23 and 24 for connection with the gas pipe line 6, andin a passage 25 between the connections 23 and 24 is formed a valve seat26 upon which seats a valve 27 fixed to the top end of the plunger 14. Apacking 28 is interposed between the cover 20 and the joint 22. Leads 29are extended from the coil 9 through the casing 16.

Referring back to FIG. 1, a thermistor 30 encapsuled in a protectivetube 31 is inserted in the supply main lb, and is coupled to the coil 9through an electrical control unit 32.

Next the mode of operation will be described hereinafter. First thecirculation pump 4 is started to circulate hot water, but thetemperature of hot water flowing through the supply main 1b is lowbecause the hot water is not sufficiently heated by the heat exchanger3. Hence the resistance of the thermistor 30 is high so that the controlunit 32 is so actuated as to flow the maximum current through the coil 9in the electromagnet valve 8 based upon the principle of the presentinvention to be described in more detail hereinafter. As a result, theplunger is moved (downwardly in FIG. 2) over the maximum distanceagainst the leaf springs 17 and 17a until the plunger moving force is inequilibrium with the forces of the leaf springs 17 and 17a so that thevalve 27 is moved away from the valve seat 26. Therefore, the maximumgas-air mixture is supplied to the gas burner 2 for the maximumcombustion. This maximum combustion may be maintained so long as theheating load is equal to or higher than the heat generated by themaximum combustion.

When the load is decreased, hot water flowing through the supply mainhas a high temperature so that the resistance of the thermistor 30 isreduced. As a result, the control unit 32 causes the electric currentflowing through the coil or solenoid 9 to decrease accordingly.Therefore the plunger 14 is caused to move upwardly in FIG. 2 under theforce of the springs 17 and 17a so that the valve 27 is now locatedcloser to the valve seat 26. The gas passage 25 is now throttled so thatthe amount of gas supplied to the gas burner 2 is decreased accordingly.

From the foregoing description, it is seen that the supply of gas to thegas burner 2 is controlled by the solenoid valve 8 in proportion to thevariation in load so that the temperature of the hot water flowingthrough the supply main lb may be maintained at the same levelindependently of the load variation. According to one of the novelfeature of the present invention, when the combustion rate is reduced toless than a predetermined level or when the gas supply is throttled orreduced to less than a predetermined level,.the electro magnetic valve 8is so actuated as to close the gas passage 25, thereby preventingbackfire. Furthermore, when the gas burner 2 is ignited again, thesupply of gas is gradually increased for a short time in order toprevent the undesired combustion or iginition noise as will be describedin more detail hereinafter.

The most important mechanical component parts of the valve 8 are meansfor supporting the plunger 14 so that the supporting means will bedescribed in more detail hereinafter. The supporting means or leafsprings 17 and 170 support the plunger 14 in such a manner that thelatter will not make contact with the bobbin 10. As a result, nofriction occurs between the plunger 14 and the bobbin 10. Furthermore,the delicate adjustment of the spacing between the valve seat 26 and thevalve 27 may be accomplished in a simple, but very reliable manner.Since the retarding force may be applied to the plunger 14 by the leafsprings 17 and 17a, other spring or the like may not be needed. This isone of the advantages in mechanical design of the present invention.

It is preferable that the lengths of the leaf springs 17 and 17a arelonger because of the following reason. It is apparent that the leadingends of the U-shaped leaf springs 17 and 17a follow not only the axialmovement of the plunger 14 but also the lateral movement thereof. Thelonger the leaf springs 17 and 17a, the lesser the lateral displacementbecomes so that the variation in spacing between the bobbin l0 and theplunger 14 may be minimized. As a consequence, the uniform magneticforce distribution may be attained so that the plunger 14 may bedisplaced in a more reliable and accurate manner.

However, when the length of the leaf springs 17 and 17a becomes toolong, the solenoid valve 8 becomes inevitably large in size. The presentinvention further provides means to overcome this problem.

Referring to FIGS. 4 and 5, a U-shaped leaf spring 33 similar to theleaf spring 17 in FIG; 3 is used. The plunger 14 is interposed betweenthe leg portions 33a and 33b of the leaf spring 33, and a pin 34 extendsbetween the free ends of the leg portions 33a and 33b. The plunger 14consists of upper and lower sections, and the root of an arm 36 isinterposed between the upper and lower sections of the plunger 14 and issecurely fixed to the lower section with a screw 35 as best shown inFIG. 5. The leading end of the arm 36 is pivoted to the pin 34. Thus theplunger 14 is indirectly supported by the leaf spring 33 at the freeends of its leg portions 33a and 33h. Opposed to the leaf spring 17shown in FIG. 3, supporting the plunger 14 at the free ends of its legportions, the plunger 14 is interposed between the leg portions 33a and33b of the U-shaped leaf spring 33 so that the length of the leafsprings may be increased without increasing the size of theelectromagnetic valve 8. When the leaf spring 33 is used for supportingthe plunger l4, the lateral displacement thereof may be furtherminimized so that the deviation of the axis of the plunger 14 from thatof the center hole of the bobbin may be minimized.

Next referring to FIGS. 6 and 7, still another plunger supporting meansin accordance with the present invention will be described. In theinstant embodiment,

spring is used, and the plunger 14 extends through the opening of theleaf spring 37 as best shown in FIG. 6. A U-shaped leaf spring 38, thesecond leaf spring, is struck out of the leaf spring 37 from one sidethereof, and the free ends of the leg portions of the U-shaped leafspring 38 are pivoted to the plunger 14 with pins 39 extendingtherefrom. The other side opposed to said one side from which theU-shaped leaf spring 38 extends, is securely fixed to the supportingmember so that the leaf spring 37 swings in the direction indicated bythe arrow a in FIG. 7 when the plunger 14 moves vertically whereas theU-shaped spring 38 swings in the direction indicated by the arrow b. Inother words, the leaf springs 37 and 38 swing in the opposite directionsso that the lateral displacement of the leaf spring 38 may be cancelledby the lateral displacement of the leaf spring 37. Therefore, theplunger 14 may be moved vertically coaxially of the center hole of thebobbin 10.

Next referring to FTG. 8, the control unit 32 will be described in moredetail hereinafter. A power source 40 and a switch 41 are connected inseries to the primary winding of a transformer 42. The secondary windingof the transformer 42 is connected to the a-c input terminals of a diodebridge 43. One end of a resistor 44 is connected to the minus ornegative output terminal of the diode bridge 43 whereas the other end isconnected to one end of a capacitor 45 whose the other end is connectedto a positive output terminal of the diode bridge 43. A series circuitconsisting of a resistor 46, a Zener diode 47 is connected in parallelwith the capacitor 45. A series circuit consisting of resistors 48, 49and 50 is connected in parallel with the Zener diode 47. In like manner,two series circuits, one consisting of a resistor 51, a variableresistor 52 and the thermistor 30 and the other consisting of resistors53 and 54 are connected in parallel with the Zener diode 47. The emitterof a transistor 55, a second active element, is connected to thejunction between the resistors 48 and 49, and the base of the transistor55 is connected to a sliding arm of the variable resistor 52. Thecollector of the transistor 55 is connected to the junction between thebase of a transistor 56, a first active element, and a resistor 5'7. Theother end of the resistor 57 and one end of a resistor 58 are connectedto the junction between the resistor 44 and the capacitor 45. The otherend of the resistor 58 is connected to the emitter of the transistor 56.The collector of the transistor 56 is connected to one junction of aparallel circuit consisting of a diode 59, the solenoid or coil 9 of theelectromagnetic valve 8, and two series resistor 60 and 61, the otherjunction of the parallel circuit being connected to the positive outputterminal of the diode bridge 43. The resistor 60 is a variable resistorwhose sliding arm is connected to the base of a transistor 63, a thirdactive element, through a resistor 62. A capacitor 64 is insertedbetween the base of the transistor 63 and the positive output terminalof the diode bridge 43. The emitter of the transistor 63 is connected tothe junction between the resistors 53 and 54, and the collector isconnected to the junction between the resistors 49 and 50 through aresistor 65. The circuit portion surrounded by the dotted line Xindicates the first control circuit and that surrounded by the dottedline Y indicates the second control circuit.

Next the mode of operation will be described. When the switch 41 isclosed so that commercial voltage is impressed across the primarywinding of the trans- 7 former 42, a low voltage is induced across thesecondary winding. The ac induced voltage is rectified by the diodebridge 43 so that a d-c voltage is applied across the capacitor 45. Theresistor 44 serves to limit the rush current when the switch 41 isclosed so that the diode bridge '43 may be prevented from being damaged.The current flows through the Zener diode 47 and the resistor 46. Inthis case, it should be noted that even when the current flowing throughthe resistor 46 varies, the voltage impressed across the three seriescircuits, first consisting of the resistors 53 and 54, the secondconsisting of the resistors 48, 49 and 50, the third consisting of theresistor 51, the variable resistor 52 and the thermistor 30, remainsunchanged because of the Zener diode 47. Even when the commercialvoltage varies, the voltage impressed across the three series circuitsremains unchanged. The resistors 48, 49 and 50, the resistor 51, thevariable resistor 52 and the thermistor 30 form the arms of a bridge inwhich the emitter and the base of the transistor 55 are connected to thejunctions between the arms of the bridge.

At the starting of the hot water boiler, cold water is flowing throughthe main 1 so that the resistanceof the thermistor 30 inserted into themain 1 is high. As a result, the base potential of the transistor 55 islower than the emitter potential so that the transistor 55 conducts. Thecurrent flows from the collector of the transistor 55 to the base of thetransistor 56 and the resistor 57 Therefore, the transistor 56 conductsso that the current flows throgh the resistor 58, the emitter andcollector of the transistor 56 to the coil 9. In this case, the currentflowing through the coil 9 is maximum so that the solenoid valve 8 isfully widely opened. As a result, the combustion rate of the gas burner2 becomes maximum.

When the load is reduced so that the temperature of the hot water rises,the resistance of the thermistor 30 is gradually decreased. When thebase potential of the transistor 55 becomes equal to the emitterpotential, the transistor 55 is shifted from the saturation region tothe active region so that the collector current is gradually decreased.As the collector current of the transistor 55 is decreased, thetransistor 56 is shifted from the saturation region into the activeregion so that the current flowing through the coil 9 is decreased. Asthe current flowing through the coil 9 is decreased, the opening of thesolenoid valve 8 is reduced so that the gas supply is controlled untilthe combustion rate attains equilibrium with the reduced load. Thetemperature of the hot water is dependent upon the voltage at which theemitter potential of the transistor 55 substantially equals the basepotential. However, since the emitter potential is almost constant, thetemperature of hot water is dependent upon the position of the slidingarm of the variable resistor 52. More particularly when the sliding armof the variable resistor 52 is in a position above the position shown inFIG. 8, the temperature of hot water is relatively high, but when thesliding arm is in a position lower than the position shown in FIG. 8,the temperature isrelatively low. Thus, the temperature of hot water maybe arbitrarily selected. It should be noted that the uneven quality ofthe thermistor 30 may be compensated by the variable resistor 52.

When the load is small, the gas supply is so controlled as to maintain asuitable combustion rate in the gas burner 2. However, the flow rate ofgas cannot'be re duced to less than a predetermined level becausebackfire occurs when the flow rate is reduced to from onehalf toone-third of the maximum gas flow rate. Therefore in the hot waterboiler of the type whose combustion rate is controlled in response tothe variation in load, the supply of gas must be interrupted when theflow rate of gas becomes less than a predetermined level. In the controlunit shown in FIG. 8, the gas flow rate may be detected by the currentflowing through the coil 9.

When the load is reduced, the temperature of hot water rises so that thegas supply is controlled in a manner described above. When the gas flowrate becomes. for example, one half of the maximum flow rate, thevoltage between the points a and b in FIG. 8 is lowered and the voltageat the points a and c become equal so that the transistor 63 conducts.The current flows through the resistor 65 and the resistor which formsone arm of the temperature detecting bridge. Therefore, the transistoris reverse biased so that the collector current of the transistor 55 isreduced. Hence the collector current of the transistor 56 which is theexciting current of the coil 9 is reduced accordingly. The reduction incollector current of the transistor 56 is felt by the transistors 63 and55 so that the current flowing through the coil 9 is immediatelyinterrupted. At the safe combustion rate of the gas burner 2, thetransistor 63 is reverse biased so that the variation in current flowingthrough the coil 9 will not influence the temperature detecting bridgecircuit. The resistor 62 is inserted in order to limit the current andthe capacitor 64 is'inserted in order to prevent the parasiticoscillation.

When the solenoid valve 8 is completely closed so that the combustion isinterrupted, the temperature of hot water is gradually lowered so thatthe resistance of the thermistor 30 is gradually increased. Therefore,the transistor 55 is gradually forward biased, and so is the transistor56. As a consequence, the transistor 63 is reverse biased. This is feltby the transistor 55 so that the latter conducts abruptly whereas thetransistor 63 is cut off. Thus the solenoid valve 8 is widely opened sothat the combustion is started again. As the temperature of hot water israised, the solenoid valve 8 is actuated in the manner described aboveto control the optimum gas flow rate.

FIG. 9 illustrates the relation between the temperature of hot water andthe current flowing through the coil 9. At the point A the combustionrate becomes one half of the maximum rate so that the solenoid valve 8is closed. At the point B the current flowing through the coil 9 risesabruptly so that the solenoid valve 8 opens wide.

Referring back to FIG. 8, the adverse effect of the conductingtransistor 63 upon the nonconducting transistor 55 for causing the sameto conduct may be eliminated when the resistance of the resistor 62 isincreased and the resistance of the resistor 50 is smaller than that ofthe resistor 49. In this case, the point B in FIG. 9 is in a relativelyupper position. However, under the reversed conditions, the point B willbe found in a lower position.

When the point B is so selected as to be above the point at which thesolenoid valve 8 is opened, the gas is gradually increased from zerowhen the gas burner is ignited again. This setting is advantageous inthat the gas burner may be ignited when the gas flow rate is still lowand then the flow rate may be rapidly increased. By

9. this so-called slow ignition, the ignition may be started veryquietly without causing any explosion noise.

The time during which the gas flow rate changes is a function of therate of temperature change and of an amplification factor of the controlcircuit.

When the control unit so far described with reference to FIG. 8 is used,backfire may be prevented and the ignition'may be started very quietly.

So'far the present invention has been described as being applied to thegas-combustion water heater for hot water heating, but it will beunderstood that the present invention may be also applied to a gasfurnace.

What is claimed is:

1. In a device of the type'in which the amount of fuel gas to besupplied to a gas burner is controlled by a proportional position typesolenoid valve,

a gas combustion control device comprising a. a first control circuitincluding a bridge circuit having a temperature-variable resistor fordetecting a medium heated by said gas burner as one arm thereof, saidcontrol circuit controlling the exciting current flowing through thesolenoid of said electromagnet valve so as to control the amount of gasfuel to be supplied to said gas burner in response to the variation inresistance of said temperaturevariable resistor;

and g a second control circuit for detecting the magnitude of saidcurrent flowing through the solenoid of said solenoid valve in such amanner that when said current decreases less than a predetermined levelthe output current from said bridge circuit is so controlled as tointerrupt said current flowing through the solenoid of saidelectromagnet valve.

2. A device as set forth in claim 1 wherein an output circuit of saidsecond control circuit for controlling said output current of saidbridge circuit is conneced'to one arm of said bridge circuit in such amanner that when said current flowing through the solenoid of saidsolenoid valve the unbalanced condition of said bridge circuit may bevaried by said output current, thereby causing the abrupt change in theoutput of said bridge circuit.

3. A device as set forth in claim 1 wherein said first and secondcontrol circuits are so coupled to each other that said current flowingthrough the solenoid of said solenoid valve may be gradually increasedin response to the temperature drop of said heated medium due to theinterruption of the gas combustion in said gas burner and that when saidcurrent reaches to said predetermined level said current may be abruptlyincreased; and

the ignition of said gas burner is made prior to said abrupt increase ofsaid current.

4. A device as set forth in claim 1 wherein said first and secondcontrol circuits are so coupled to each other that said current may begradually decreased in response to the temperature rise of said heatedmedium and that said current may be abruptly decreased when said currentreaches said predetermined level.

5. A device as set forth in claim ll wherein said first control circuitfor controlling the exciting current comprises 7 said bridge circuit anda first active element which is controlled by asecond active elementforming the output circuit of said bridge circuit, the output circuit ofsaid first active element being connected in series to the solenoid ofsaid solenoid valve; and

said second control circuit comprises a detector circuit for detectingthe voltage applied across the solenoid of said solenoid valve, and

a third active element which is controlled in response to said detectedvoltage,

the output of said third active element being so connected to one arm ofsaid bridge circuit that the variation in output current of said outputcircuit may cause the variation in bias voltage applied to said secondactive element in said bridge circuit.

6. A device as set forth in claim 1 wherein said proportional actiontype solenoid valve comprises said solenoid, a plunger and spring meansfor supporting said plunger.

7. A device as set forth in claim 6 wherein said spring means compriseleaf springs,

one end of each of said leaf springs being securely fixed to astationary member of said device whereas the other end is bifurcated,

the bifurcated portions of said leaf spring being extended laterally ofsaid plunger away from said one fixed end,

the leading ends of said bifurcated portions of said leaf springsupporting a rigid arm extending from said plunger.

8. A device as set forth in claim 6 wherein said spring means comprisesfirst leaf springs,

one end of each of said leaf springs being securely fixed to astationary member of said device,

the other end of said leaf spring being bifurcated,

the bifurcated portions of said leaf spring being extended laterally ofsaid plunger away from said fixed one end,

second leaf springs being fixed to the leading ends of said bifurcatedportions and extended toward said fixed one end of said first leafspring,

the leading ends of said second leaf springs supporting said plunger.

9. A device as set forth in claim 6 wherein said spring means havingadjusting means of reaction force thereof.

i t it: it a

1. In a device of the type in which the amount of fuel gas to besupplied to a gas burner is controlled by a proportional position typesolenoid valve, a gas combustion control device comprising a. a firstcontrol circuit including a bridge circuit having a temperature-variableresistor for detecting a medium heated by said gas burner as one armthereof, said control circuit controlling the exciting current flowingthrough the solenoid of said electromagnet valve so as to control theamount of gas fuel to be supplied to said gas burner in response to thevariation in resistance of said temperature-variable resistor; and asecond control circuit for detecting the magnitude of said currentflowing through the solenoid of said solenoid valve in such a mannerthat when said current decreases less than a predetermined level theoutput current from said bridge circuit is so controlled as to interruptsaid current flowing through the solenoid of said electromagnet valve.2. A DEVICE AS SET FORTH IN CLAIM - WHEREIN AN OUTPUT CIRCUIT OF SAIDSECOND CONTROL CIRCUIT FOR CONTROLLING SAID OUTPUT CURRENT OF SAIDBRIDGE CIRCUIT IS CONNECTED TO ONE ARM OF SAID BRIDGE CIRCUIT IN SUCH AMANNER THAT WHEN SAID CURRENT FLOWING THROUGH THE SOLENOID OF SAIDSOLENOID VALVE THE UNBALANCED CONDITION OF SAID BRIDGE CIRCUIT MAY BEVARIED BY SAID OUTPUT CURRENT, THERBY CAUSING THE ABRUPT CHANGE IN THEOUTPUT OF SAID BRIDGE CIRCUIT. -. A DEVICE AS SET FORTH IN CLAIM -WHEREIN SAID FIRST AND SECOND CONTROL CIRCUITS ARE SO COUPLED TO EACHOTHER THAT SAID CURRENT FLOWING THROUGH THE SOLENOID OF SAID SOLENOIDVALVE MAY BE GRADUALLY INCREASED IN RESPONSE TO THE TEMPERATURE DROP OFSAID HEATED MEDIUM DUE TO THE INTERRUPTION OF THE GAS COMBUSTION IN SAIDGAS BURNER AND THAT WHEN SAID CURRENT REACHES TO SAID PREDETERMINEDLEVEL SAID CURRENT MAY BE ABRUPTLY INCREASED: AND THE IGNITION OF SAIDGAS BURNER IS MADE PRIOR TO SAID ABRUPT INCREASE OF SAID CURRENT. $. Adevice as set forth in claim 1 wherein said first and second controlcircuits are so coupled to each other that said current may be graduallydecreased in response to the temperature rise of said heated medium andthAt said current may be abruptly decreased when said current reachessaid predetermined level.
 5. A DEVICE AS SET FORTH IN CLAIM - WHEREINSAID FIRST CONTROL CIRCUIT FOR CONTROLLING THE EXCITING CURRENTCOMPRISES SAID BRIDGE CIRCUIT AND A FIRST ACTIVE ELEMENT WHICH ISCONTROLLED BY A SECOND ACTIVE ELEMENT FORMING THE OUTPUT CIRCUIT OF SAIDBRIDGE CIRCUIT, THE OUTPUT CIRCUIT OF SAID FIRST ACTIVE ELEMENT BEINGCONNECTED IN SERIES TO THE SOLENOID OF SAID SOLENOID VALVE: AND SAIDSECOND CONTROL CIRCUIT COMPRISES A DETECTOR CIRCUIT FOR DETECTING THEVOLTAGE APPLIED ACROSS THE SOLENOID OF SAID SOLENOID VALVE, AND A THIRDACTIVE ELEMENT WHICH IS CONTROLLED IN RESPONSE TO SAID DETECTED VOLTAGE,THE OUTPUT OF SAID THIRD ACTIVE ELEMENT BEING SOCONNECTED TO ONE ARM OFSAID BRIDGE CIRCUIT THAT THE VARIATION IN OUTPUT CURRENT OF SAID OUTPUTCIRCUIT MAY CAUSE THE VARIATION IN BIAS VOLTAGE APPLIES TO SAID SECONDACTIVE ELEMENT IN SAID BRIDGE CIRCUIT. ". A device as set forth in claim1 wherein said proportional action type solenoid valve comprises saidsolenoid, a plunger and spring means for supporting said plunger.
 7. Adevice as set forth in claim 6 wherein said spring means comprise leafsprings, one end of each of said leaf springs being securely fixed to astationary member of said device whereas the other end is bifurcated,the bifurcated portions of said leaf spring being extended laterally ofsaid plunger away from said one fixed end, p1 the leading ends of saidbifurcated portions of said leaf spring supporting a rigid arm extendingfrom said plunger.
 8. A device as set forth in claim 6 wherein saidspring means comrises first leaf springs, one end of each of said leafsprings being securely fixed to a stationary member of said device, theother end of said leaf spring being bifurcated, the bifurcated portionsof said leaf spring being extended laterally of said plunger awayfromsaid fixed one end, second leaf springs being fixed to the leadingends of said bifurcated portions and extended toward said fixed one endof said first leaf spring, the leading ends of said second leaf springssupporting said plunger.
 9. A device as set forth in claim 6 whereinsaid spring means having adjusting means of reaction force thereof.